Broadband services supplied to a customer over the copper local loop are becoming increasingly more popular with the rise of the Internet and other technologies requiring wideband local loop applications. It is well known that Digital Subscriber Line (DSL) technology improves the bandwidth of existing analog phone systems. Data throughput of up to 52 Mbits/sec can be provided over small distances, which increase as the data rate is lowered.
The customer local loop refers to the existing twisted-pair wire that extends between a local telephone company switching office and most homes and offices. As is well known, the bandwidth was typically limited to 3,000 Hz, because of the relegation of the local twisted-pair wire to the voice telephone system and its audio frequencies. In the past, most telephone switching equipment was designed to cut off signals from about 4,000 Hz and filter noise off the voice line.
More phone companies are upgrading their switching equipment to obtain a greater bandwidth since the advent of the Internet. The different technologies of DSL, also referred to as xDSL, range in speed from 16K bits/sec to 52 Mbits/sec, and can be either symmetrical, where traffic flows at the same speed in both directions or asymmetrical, where the downstream capacity is higher than the upstream capacity. Asymmetrical DSL services can typically be used by Internet users at home, for example, allowing a user to download more graphic files and upload only commands.
As the data rate increases, the carrying distance for xDSL service decreases. Also, xDSL connections are point-to-point and are always connected with no dial up, and no switching. There is always a direct connection into a carrier's frame relay, ATM (Asynchronous Transfer Mode), or an Internet-connect system.
The different types of xDSL service include High-bit-rate Digital Subscriber Line (HDSL), which provides T1 data rates of 1.544 Mbits/sec over about 12,000 feet of line length. Two lines are used and voice services are not operable. It is usually provided for feeder lines, interexchange connections, Internet servers, and private data networks.
Symmetrical Digital Subscriber Line (SDSL) is a symmetrical, bidirectional DSL service using one twisted-pair wire and operates above the voice frequency. This allows voice and data to be carried on the same wire.
Asymmetrical Digital Subscriber Line (ADSL) allows a much greater downstream data rate. It is operable best for Internet services and the rate varies, depending on the downstream rate and downstream distance. For example, when using a downstream rate of 1.544 Mbits/sec, the downstream maximum line distance is about 18,000 feet. If the downstream rate is increased to 8.448 Mbits/sec, the downstream maximum line distance is only about 9,000 feet.
Very high-bit-rate Digital Subscriber Line (VDSL) is a very high asymmetrical data rate. It allows an upstream rate of about 12.96 Mbits/sec with a maximum line distance of about 4,500 feet, and an upstream rate of about 51.84 Mbits/sec, with an upstream maximum line distance of about 1,000 feet.
Rate-Adaptive Digital Subscriber Line (RADSL) is similar to ADSL, but includes a rate-adapted feature to adjust transmission speed to match the quality of the line and length of the line. It is possible to use a line-pulling technique to establish a connection speed when the line is first established.
ISDN DSL (IDSL) operates at about 128 Kbps, which is less than most other DSL technologies. It is a dedicated service as compared to standard ISDN services. IDSL is data-only and lacks any analog voice line.
Although xDSL technologies are becoming increasingly important, there is still an inability to adequately prequalify the local copper loops accurately. This has been a significant obstacle for the Local Exchange Carriers (LECS). Prequalification has now become critical because the different xDSL technology services is dependent on the design and quality of the outside plant (OSP) and the presence of load coils, which block DSL transmission. Prequalification also determines if the local loop is capable of supporting DSL transmission prior to any attempt to provide service. There will be significant cost savings for the LEC if the loop could be qualified without having to dispatch technicians to either a central office (CO) or to the customer premises. As noted above, there are a number of DSL services and even more are projected by the industry. Thus, there is a strong need for an even more improved automated testing capability to handle the growing line volume of xDSL technologies. It is necessary, then, to predict a local loop's capability to support xDSL services across an entire range of frequencies over which this technology can operate.
There are some systems for estimating the ability of a subscriber loop to support broadband services, such as disclosed in U.S. Pat. No. 6,091,713 to Lechleter et al. Also, there are various Wideband Test Packs (WTP) and Remote Test Units (RTU), such as manufactured by Harris Corporation of Melbourne, Fla., that are used for diagnosing service-affecting problems for all xDSL and ISDN services. These units can act as an intelligent test head, as known to those skilled in the art. Greater efficiency in testing, qualifying, and quantifying the local loop is desired, however.